Fig. 1: Ultrasonic Pulse Velocity Meter
The apparatus for ultrasonic pulse velocity measurement shall consist of the following:
a) Electrical pulse generator,
b) Transducer - one pair,
c) Amplifier, and
d) Electronic timing device.
The ultrasonic pulse velocity method could
be used to establish:
(i) the homogeneity of the concrete,
(ii) the presence of cracks, voids and other imperfections,
(iii) changes in the structure of the concrete which may occur with time,
(iv) the quality of the concrete in relation to standard requirements,
(v) the quality of one element of concrete in relation to another, and
(vi) the values of dynamic elastic modulus of the concrete.
The ultrasonic pulse is generated by an
electroacoustical transducer. When the pulse is
induced into the concrete from a transducer, it
undergoes multiple reflections at the boundaries
of the different material phases within the concrete.
A complex system of stress waves is
developed which includes longitudinal (compressional), shear (transverse) and surface
(rayleigh) waves. The receiving transducer
detects the onset of the longitudinal waves,
which is the fastest.
Because the velocity of the pulses is almost independent of the geometry of the material through which they pass and depends only on its elastic properties, pulse velocity method is a convenient technique for investigating structural concrete.
The underlying principle of assessing the quality of concrete is that comparatively higher velocities are obtained when the quality of concrete in terms of density, homogeneity and uniformity is good. In case of poorer quality, lower velocities are obtained. If there is a crack, void or flaw inside the concrete which comes in the way of transmission of the pulses, the pulse strength is attenuated and it passes around the discontinuity, thereby making the path length longer. Consequently, lower velocities are obtained. The actual pulse velocity obtained depends primarily upon the materials and mix proportions of concrete. Density and modulus of elasticity of aggregate also significantly affect the puise velocity.
IS-13311 (Part 1):1992 (Reaffirmed- May 2013) “Non-Destructive Testing of Concrete- Methods of Test (Ultrasonic Pulse Velocity)"
V = L/T
Smoothness of contact surface under test affects
the measurement of ultrasonic pulse velocity.
For most concrete surfaces, the finish is usually
sufficiently smooth to ensure good acoustical
contact by the use of a coupling medium and
by pressing the transducer against the concrete
surface. When the concrete surface is rough and
uneven, it is necessary to smoothen the surface
to make the pulse velocity measurement
In general, pulse velocity through concrete increases with increased moisture content of concrete. This influence is more for low strength concrete than high strength concrete. The pulse velocity of saturated concrete may be up to 2 percent higher than that of similar dry concrete. In general, drying of concrete may result in somewhat lower pulse velocity.
As concrete is inherently heterogeneous, it is
essential that path lengths be sufficiently long
so as to avoid any error introduced due to its
heterogeneity. In field work, this does not pose
any difficulty as the pulse velocity measurements
are carried out on thick structural concrete
members. However, in the laboratory where
generally small specimens are used, the path
length can affect the pulse velocity readings.
The shape and size of the concrete member do not influence the pulse velocity unless the least lateral dimension is less than a certain minimum value, for example the minimum lateral dimension of about 80 mm for 50 kHz natural frequency of the transducer. Table 1 gives the guidance on the choice of the transducer natural frequency for different path lengths and minimum transverse dimensions of the concrete members.
Variations of the concrete temperature between 5 and 30°C do not significantly affect the pulse velocity measurements in concrete. At temperatures between 30 to 60°C there can be reduction in pulse velocity up to 5 percent. Below freezing temperature, the free water freezes within concrete, resulting in an increase .in pulse velocity up to 7.5 percent.
When concrete is subjected to a stress which is
abnormally high for the quality of the concrete,
the pulse velocity may be reduced due to the
development of micro-cracks. This influence is
likely to be the greatest when the pulse path is
normal to the predominant direction of the
planes of such micro-cracks. This occurs when
the pulse path is perpendicular to the direction
of a uniaxial compressive stress in a member.
This influence is generally insignificant unless the stress is greater than about 60 percent of the ultimate strength of the concrete.
The pulse velocity measured in reinforced concrete
in the vicinity of reinforcing bars is
usually higher than in plain concrete of the
same composition. This is because, the pulse
velocity in steel is 1.2 to 1.9 times the velocity
in plain concrete and, under certain conditions,
the first pulse to arrive at the receiving transducer
travels partly in concrete and partly in
The apparent increase in pulse velocity depends upon the proximity of the measurements to the reinforcing bar, the diameter and number of the bars and their orientation with respect to the path of propagation.
|S. No.||Pulse velocity by Cross Probing (km/sec)||Concrete Quality Grading|
|2||3.5 to 4.5||Good|
|3||3.0 to 3.5||Medium|
Table 1 : Velocity Criterion for Concrete Quality Grading.